In recent years, hybrid and electric vehicles have been rapidly developed. In line with this, various suggestions have been made to change the braking system of vehicles from the conventional mechanical hydraulic control to electrohydraulic control.
The electrohydraulic control of vehicles is generally performed using a battery-operated power supply. Therefore, if the power supply from the battery is cut off for some reason, the hydraulic control is shut off, possibly making it impossible to brake the vehicles. To prepare for such an emergency, various vehicle power supply devices have been proposed which include high-capacity capacitors or the like as an auxiliary power supply besides the battery.
Since vehicle power supply devices involve the vehicle braking in an emergency, it is crucial for them to ensure the provision of electricity in an emergency by accurately determining degradation of capacitors, which are their key devices.
In an attempt to achieve this, a conventional vehicle power supply device determines degradation of the capacitors as follows. First, the vehicle power supply device obtains the internal resistance value and the capacitance value of a capacitor unit including a plurality of capacitors. Then, the device makes the temperature sensor disposed near the capacitor unit detect the temperature. Next, the device corrects the internal resistance value and the capacitance value based on the detected temperature. Finally, the device compares these corrected values with the degradation standard value data corresponding to the detected temperature so as to determine the degradation. More specifically, the internal resistance value of the capacitor unit is correlated with the inverse of the capacitance value when sufficient electricity is supplied to the load; however, the correlation changes when the capacitor unit is degraded. This fact is taken advantage of to determine the degradation of the vehicle power supply device as follows. The relation between the internal resistance value and the capacitance value when the capacitor unit is degraded is previously stored in association with each temperature in a ROM connected to a controller (microcomputer). The device is determined to be degraded when the internal resistance value corresponding to the capacitance value that is corrected based on the current temperature reaches the stored degradation standard value of the internal resistance value.
FIG. 7 is a correlation graph between a capacitance value and the degradation standard value of an internal resistance value of the capacitor unit of a conventional vehicle power supply device, using degradation determination formulas corresponding to different temperatures. In other words, the correlation graph shows an example of the degradation standard value data of the internal resistance value corresponding to the capacitance value. In FIG. 7, horizontal axis represents the capacitance, and the vertical axis represents the internal resistance. The correlation between them differs depending on the temperature and is shown at temperatures at 15° C. intervals between −30° C. and 30° C.
When the capacitor unit has a temperature of 0° C., a capacitance of 10 F and an internal resistance of 130 mO, the degradation standard value (limit value) is 230 mO as shown by the circle plot (0° C.) of FIG. 7. Since the current internal resistance (130 mO) has not reached the standard value, the capacitor unit is determined not to be degraded yet.
When the capacitor unit is at 15° C., 11 F and 115 mO, the degradation standard value (limit value) is 180 mO as shown by the square plot (15° C.) of FIG. 7. Since the current internal resistance (115 mO) has not reached the standard value, the capacitor unit is determined not to be degraded yet.
On the other hand, when the capacitor unit is at 30° C., 11 F and 110 mO, the degradation standard value (limit value) is 80 mO as shown by the “X” plot (30° C.) of FIG. 7. Since the current internal resistance (110 mO) has exceeded the standard value, the capacitor unit is determined to be degraded.
One of the prior arts related to the present application is disclosed in Japanese Patent Unexamined Publication No. 2005-28908.
It is understood, as described above, that the conventional vehicle power supply device can determine degradation of the capacitor unit and that the determination is accurate because it is performed with respect to each temperature.
However, the degradation standard value data that can be stored in the ROM is limited to the amount corresponding to the plots shown in FIG. 7 due to the storage capacity of the ROM. Therefore, when the plot corresponding to the temperature exists, the degradation can be determined with high accuracy. On the other hand, when the plot corresponding to the temperature does not exist, the capacitance value on the plot corresponding to the closest temperature is used. Since this is not the accurate degradation standard value, the degradation cannot be determined with sufficient accuracy when the plot corresponding to the temperature does not exist.